Figure 1. Sample locations and local annual
photoperiod-temperature ellipsoids. (A) Distribution map of localities
of common vole samples used in this study. Location numbers with
corresponding environmental proxies are listed in Table S1. (B) Annual
patterns (counter-clockwise) of photoperiod and 10-year average monthly
ambient temperature for each sample location in Western Europe and (C)
in Eastern Europe. Temperature data were obtained from the closest
weather station (always within 110 km of sample location) obtained from
Wunderground (https://www.wunderground.com/). Photoperiod was
obtained from https://www.timeanddate.com, and is based on civil
twilight times at dawn and dusk, which is the timing at which log light
intensities change most rapidly (Daan & Aschoff, 1975; Hut et al.,
2013). Civil twilight incorporates geographical and seasonal variation
in the duration of twilight (Nielsen, 1961), and is therefore considered
as the moment of ‘lights on’ and ‘lights off’ for biological systems
(Hut et al., 2013). Dotted lines indicate a temperature threshold at
6.6°C from which the corresponding predicted critical photoperiod (pCPP)
in spring (ellipse crossing the 6.6 °C line for the second time) is used
as a proxy for onset of grass growth and consequently as a proxy
for onset of the favorable season. Regional pCPP’s are listed in Table
S1. Colors indicate latitude, ranging from 42°N (yellow) to 59°N
(purple).